<?php
use yii\widgets\ActiveForm;
?>

<?php $form = ActiveForm::begin() ?>

<?= $form->field($model, 'files')->widget('yidashi\webuploader\Webuploader',['options'=>['boxId' => 'picker', 'previewWidth'=>200, 'previewHeight'=>150]]);

?>

<button>Submit</button>

<?php ActiveForm::end() ?>